FIG. 1A shows a sectional side view of a prior art capacitive pressure transducer 100. For convenience of illustration, FIG. 1A, as well as other figures in the present disclosure, are not drawn to scale. As shown, transducer 100 includes a housing 102, a capacitive pressure sensor 106 disposed within housing 102, an inlet tube 104, and a filtering mechanism 108. For convenience of illustration, many details of transducer 100 are omitted from FIG. 1A. However, such sensors are well known and are described for example in U.S. Pat. Nos. 5,911,162 and 6,105,436 and U.S. patent application Ser. Nos. 09/394,804 and 09/637,980.
Briefly, transducer 100 is normally coupled to a gas line, or some other external source of gas or fluid, 110 by a coupling 112. In operation, sensor 106 generates an output signal representative of the pressure within the external source 110.
Pressure transducers such as transducer 100 are often used in integrated circuit fabrication foundries, for example, to measure the pressure of a fluid in a gas line that is being delivered to a deposition chamber, or to measure the pressure within the deposition chamber itself. Some of the processes used in integrated circuit fabrication, such as the etching of aluminum, tend to generate a large volume of particles or contaminants. It is generally desirable to prevent such contaminants from entering the sensor 106. When contaminants do enter, or become built up in, sensor 106 the accuracy of the pressure measurement provided by transducer 100 is adversely affected. Accordingly, prior art pressure transducers have used a variety of mechanisms to prevent contaminants from reaching the sensor 106. Such prior art filtering mechanisms are generally permanently fixed to the housing 102 of the transducer, are disposed between the inlet tube 104 and the sensor 106, and are indicated generally in FIG. 1A at 108.
FIG. 1B shows a more detailed view of a particular prior art pressure transducer 100 showing both the sensor 106 (which as discussed below includes elements 120, 122, 124) and the filtration mechanism 108 (which as discussed below includes elements 140, 150). The housing of transducer 100 includes two housing members 102a and 102b, which are separated by a relatively thin, flexible conducting diaphragm 160. Housing member 102b and diaphragm 160 define a sealed interior chamber 120. Housing member 102a and diaphragm 160 define an interior chamber 130 that opens into inlet tube 104. Diaphragm 160 is mounted so that it flexes, or deflects, in response to pressure differentials in chambers 120, 130.
Transducer 100 includes an electrode disk 122 disposed within chamber 120. Electrode disk 122 is supported within chamber 122 by a support 124. One or more conductors 126 are disposed on the bottom of electrode disk 122 and conductor 126 is generally parallel to and spaced apart from diaphragm 160. Diaphragm 160 and conductor 126 define a capacitor 128. The capacitance of capacitor 128 is determined in part by the gap between diaphragm 160 and conductor 126. As diaphragm 160 flexes in response to changes in the differential pressure between chambers 120, 130, the capacitance of capacitor 128 changes and thereby provides an indication of the differential pressure.
In operation, a reference pressure, which may be near vacuum, is normally provided in chamber 120, inlet tube 104 is connected via coupling 112 to a gas line, or other external source of fluid or gas, 110, and transducer 100 provides a signal indicative of the pressure in the external source. In other configurations, a second inlet tube leading into chamber 120 may be provided and connected to a second external source. In such configurations, transducer 100 provides a signal indicative of the differential pressure between the two external sources. In the illustrated transducer, the pressure sensor includes generally capacitor 128 as well as the electrode disk 122 and the structural members 124 used to support the electrode.
In the illustrated transducer, the contaminant filtration mechanisms include a particle trap system 140 and a baffle 150. Trap system 140 includes a baffle 141, a top view of which is shown in FIG. 2. Baffle 141 includes a central, circular, closed portion 142 and an annular region, defining a plurality of openings 144, disposed around closed portion 142. Openings 144 are formed as series of sectors evenly spaced about the baffle 141 in a circumferential direction, and are also arranged at different diameters radially. The diameter of central portion 142 is greater than that of inlet 104 and thereby blocks any direct paths from inlet tube 104 to the diaphragm 160. So, any contaminant in inlet tube 104 traveling towards diaphragm 160 can not follow a straight line path and must instead, after traveling the length of inlet tube 104, then travel in a direction generally perpendicular to the length of inlet tube 104 (the perpendicular direction being generally illustrated in FIG. 1B by the arrow L), enter an annular chamber region 146, and then pass through one of the peripheral openings 144. The peripheral openings 144 are sized to prevent relatively large particles (e.g., 250 microns and larger) from passing through the openings. Trap system 140 also includes the chamber 146, which is defined between baffle 141 and housing member 102a. Particles that can't pass through openings 144 tend to accumulate in, or become trapped in, chamber 146.
As noted above, transducer 100 also includes a baffle 150 to prevent contaminants from reaching the diaphragm 160. Baffle 150 is described in copending U.S. patent application Ser. No. 09/394,804. FIG. 3 shows a top view of baffle 150. As shown, baffle 150 is essentially a circular metal plate with a plurality of evenly spaced tabs 152 disposed about the circumference. Housing member 102a has stepped regions that come in contact with tabs 152 so as to support baffle 150 in the position shown in FIG. 1B.
Tabs 152 essentially define a plurality of annular sectors 154 (shown in FIGS. 1B and 3) having a width in the radial direction between the peripheral edge of baffle 150 and housing member 102a that is determined by the length of the tabs. Baffle 150 and housing member 102a define a region 158 through which any molecule must flow if it is to travel from inlet tube 104 to diaphragm 160. The region 158 is annular and is bounded above by baffle 150 and below by either baffle 141 or housing member 102a (where the terms “above” and “below” are with reference to FIG. 1B, but do not imply any absolute orientation of transducer 100). Molecules may enter region 158 via the peripheral openings 144 and may exit region 158 via the annular sectors 154 (shown in FIGS. 1B and 3) between the peripheral edge of baffle 150 and housing member 102a. 
Region 158 is characterized by a length L and a gap g. The length L of region 158 (shown in FIG. 1B) is the distance between openings 144 and annular sectors 154. The gap g of region 154 is the distance between baffle 150 and housing member 102a. The aspect ratio of region 158 is defined as the ratio of a length L to the gap g. As taught in U.S. patent application Ser. No. 09/394,804, the aspect ratio is preferably greater than 50. The length L is preferably at least 1 cm, and preferably in the range of about 1–4 cm; the gap g is preferably no more than about 0.1 cm, and preferably in a range of about 0.025–0.1 cm.
When the pressure in chamber 130 is relatively low (e.g., less than 0.02 Torr), movement of material in chamber 130 is characterized by “molecular flow”. Under such conditions, any molecule traveling through region 158 will likely collide with the surfaces of baffle 150 and housing member 102a many times prior to reaching, and passing through, an annular sector 154. The probability that a contaminant particle will become deposited on, or stuck to, a surface of baffle 150 or housing member 102a rather than continuing on through region 158 and passing thorough an annular sector 154 is an increasing function of the number of collisions the particle makes with the surfaces of baffle 150 and housing member 102a. Selecting the aspect ratio of the length L to the gap g to be greater than 50 insures that any contaminant traveling through region 158 is likely to become deposited on a surface of either baffle 150 or housing member 102a rather than continuing on through region 158, passing through an annular sector 154, and ultimately reaching the diaphragm 160.
In general, the goal of filtration mechanisms 108 (FIG. 1A), such as trap system 140 or baffle 150 (FIG. 1B) is to prevent contaminants from reaching the pressure sensor 106 (FIG. 1A). The use of trap system 140 and baffle 150 has been extremely effective at reducing the number of contaminants that reach the pressure sensor 106, and in particular from reaching diaphragm 160. However, some processes generate such a large volume of contaminants that even greater ability to filter out contaminants and reduce the likelihood that contaminants will reach the pressure sensor is desirable.